1. Field of the Invention
The present invention relates to a color projection television system which forms an image on a screen by magnifying and synthesizing images of the component colors, such as three primary colors, displayed on respective unicolor display elements, such as cathode-ray tubes or liquid crystal display units.
2. Background of the Invention
Projection television systems are now finding more domestic and industrial applications. An example of prior art projection television system is shown in FIG. 1 which is a view as seen from above. As illustrated, it comprises a screen 30 having a front surface 31 and a rear surface 32, on which a color image is formed by magnifying and synthesizing unicolor images displayed on a green cathode-ray tube 10G, a red cathode-ray tube 10R, and a blue cathode-ray tube 10B. The screen 30 has a transmittance, and the image projected on the rear surface 32 is seen on the front surface 31. Viewers 40 sit facing the front surface 31 and see the image on the front surface 31. The cathode-ray tubes 10G, 10R and 10B are disposed side by side. The green cathode-ray tube 10G is at the center, and the red cathode-ray tube 10R and the blue cathode-ray tube 10B are on the respective sides (the right side (upper side as seen in the figure) and the left side (lower side as seen in the figure) as seen from the viewers 40) of the green cathode-ray tube 10G. The light rays forming the images from the three cathode-ray tubes 10G, 10R and 10B are converged by respective lenses 20G, 20R and 20B, and focused on the rear surface 32 of the screen 30, and a full-color image is formed on the rear surface 32 of the screen 30. The lens 20G is so disposed that its optical axis 20Gx is normal to the surface of the screen 30. The lenses 20R and 20B are so disposed that their optical axes 20Rx and 20Bx are at an angle (converging angle) .theta. with respect to a normal line on the surface of the screen 30. That is, the lens surfaces are at an angle .theta. with respect to the surface of the screen 30.
A problem associated with the prior art projection system described above is a color imbalance that is caused by the asymmetry in brightness distribution of each of the component colors, i.e., red and blue from the cathode-ray tubes oriented obliquely. That is, the brightness of the red image is greater on the left side 33 than on the right side 34, and the brightness of the blue image is greater on the right side 34 than on the left side 33. When the intensity of the images displayed on the cathode-ray tubes are so adjusted that a color balance is achieved at the center of the screen, the color imbalance of opposite tendencies appear on the opposite sides of the screen.
One reason for the asymmetric distribution of each of the red and blue images is that the field angles .omega..sub.1 and .omega..sub.2 on the respective sides differ from each other. As is known, the density of light flux (rays) is decreased with the angle formed by the light ray reaching the point in question and the optical axis. This leads to the asymmetric distribution of the brightness (illuminance) on the rear surface 32 of the screen 30, and hence the brightness (luminance) of the image as seen on the front surface 31 of the screen 30.
Further description on the asymmetric distribution of the brightness will be given with reference to FIG. 2.
As shown In FIG. 2, the brightness of the green image is maximum at the center 30C of the screen 30 and decreases with the distance from the center 30C, symmetrically about the center. The brightness of the red and blue images are asymmetrical about the center 30C. The brightness of the red and blue images as normalized with respect to the brightness of the green image, at the respective points on the screen, is as shown in FIG. 3. The brightness of the red image as normalized with reference to the brightness of the green image increases toward the left end 35 and decreases toward the right end 36. The brightness of the blue image as normalized with reference to the brightness of the green image increases toward the right end 36 and decreases toward the left end 35. The normalized brightness can be used as a basis on which to make judgement as to whether or not the color balance is attained, or as a parameter reflecting color balance or imbalance.
As is better seen from FIG. 4, the difference between field angles is greater between the corners of the screen. For instance, the normalized brightness of the red image increases toward the upper-left corner 37a and toward the lower-left corner 37c, and decreases toward the upper-right corner 37b and toward the lower-right corner 37d. The normalized brightness of the blue image increases toward the upper-right corner 37b and toward the lower-right corner 37d, and decreases toward the upper-left corner 37a and lower-left corner 37c. The field angles of the red image at the upper-left corner 37a and the upper-right corner 37b are indicated by .omega..sub.37a and .omega..sub.37b.
The angle of convergence, .theta., is increased as the distance from the screen to the cathode-ray tubes is decreased for reduction in size of the system. Thus, the above problem imposes a limitation to the size reduction.
Another reason for tile asymmetric distribution of the brightness (luminance) of the image of each color on the screen as seen from the viewers on the front side 31 of the screen 30, is that the transmittance of the screen 30 differs depending on the angle of incidence of light.
A method for compensating for the color imbalance is to partially increase or decrease the beam drive current (which determines the intensity of the beam impinging the fluorescent surface In the cathode-ray tube) thereby to vary the brightness of the image on the cathode-ray tubes. But increasing the beam drive current increases the size of the beam spot on the fluorescent surface, resulting in degradation in the resolution. On the other hand, partially decreasing the beam drive current decreases the average brightness of the Image on the screen.